This invention relates to electronic devices, and, more particularly, to integrated circuits that combine gallium arsenide and silicon device technologies on a single substrate.
Electronic technology has been developed to permit the construction of ever-smaller devices and arrays of devices. In integrated circuits, many miniaturized active and passive elements are placed upon a single substrate, that is often less than an inch square in size. The reduction in size permits an increase in speed of operation and a decrease in power consumption, and has led to the development of computers and other machinery having extraordinary capability.
Many of the miniature active electronic devices, such as transistors, that are formed on the substrate of an integrated device are fabricated from silicon and doped silicon. Such silicon-based technology has been advanced to submicron levels, so that thousands of devices can be packed onto a single substrate to achieve very high packing densities. Silicon devices are particularly effective for processing electronic signals having relatively low frequency, on the order of less than about 1 Gigahertz, and which do not produce optical signals, resulting in limitations on the applications of silicon-based integrated circuits.
Similar types of devices, but based upon gallium arsenide and doped gallium arsenide as the active material, can be used to process electronic signals of higher frequencies, such as 10-20 Gigahertz, as well as optical and optoelectronic signals. However, due to power and yield limitations, the gallium arsenide devices cannot achieve the packing densities of the silicon circuits. A number of specialized microelectronic circuits and optical devices are available for processing optical and electrical high frequency signals, and are now in use.
As indicated, silicon-based technologies and gallium arsenide-based technologies each have particular limitations in their application. Some of the limitations, such as the optimal functioning in various frequency ranges, stem from the nature of the materials themselves. As such, it is not expected that the respective limitations could be readily overcome by further development of the particular material.
Hybrid systems have been proposed and constructed to incorporate the most useful characteristics of both silicon-based technologies and gallium arsenide based technologies, in a single apparatus. In such hybrid systems, both silicon-based and gallium arsenide-based active integrated circuits are used, and are placed into a single package. The individual integrated circuits are linked by conductors, such as wires, that are attached to various input and output terminals of the integrated circuits. The linked individual integrated circuits can thereby interact with each other, with signals processed in one circuit used as input to other circuits in complex ways.
While such hybrid systems are operable, they are not optimal in the sense that the necessary long interconnections between devices slow the operation of the system. They also reduce its reliability, and certainly increase its size. The cost of such systems is high due to the low fabrication throughput resulting from the individual processing of devices. There therefore exists a need for improvements to such hybrid systems that overcome these difficulties, while retaining the various advantages of the different technologies.
There have been developments which are potentially useful in meeting this need. Gallium arsenide has been grown as epitaxial layers on bulk silicon single crystals. It is conceivable that hybrid systems could be built up on a single slice of silicon, with some circuits in the silicon and others in the deposited gallium arsenide. However, this result cannot readily be applied practically to making hybrid circuits, as the bulk silicon is a conductor of electricity and cannot provide the needed electrical isolation of the different circuit elements. In addition, the bulk silicon substrate cannot be effectively used for transmission of high frequency signals due to the loss factor associated with its dielectric constant.
There is at present no satisfactory solution for the problem of fabricating hybrid systems that incorporate both high frequency or optoelectronic gallium arsenide technology and high density silicon technology. The need for a compatible approach remains. Ideally, the approach would permit the use of all of the techniques developed for the two technologies in an optimized fashion, yet also permit reduced size and power consumption, and increased speed of operation and reliability. The present invention fulfills this need, and further provides related advantages.